A non-contact, ultrasonic vibrometer can include an off-the-shelf, pulse-echo, focused sensor, similar to those used in medical ultrasonics and non-destructive testing, but adapted for use in air. Typically the focused beam has a stand-off distance of a few inches. It is known that a beam can measure the distance to a surface quite accurately, as described in                1. R. Hickling and S. P. Marin, “The use of ultrasonics for gauging and proximity sensing in air”, J. Acoust. Soc. Amer., 79(4), 1151-1160, 1986.A previous acoustic-seismic method is described in:        2. J. M. Sabatier and K. E. Gilbert, “Method of detecting buried objects by measuring seismic vibrations by coupling with a remote source of sound”, U.S. Pat. No. 6,081,481, Jun. 27, 2000.in which continuous sound penetrates the ground, returning echoes from a buried object. A laser-Doppler vibrometer (LDV) scans the surface of the ground to detect seismic vibrations at the surface resulting from the echoes. The LDV emits a laser beam which detects the Doppler shift of the reflected light due to the motion of the surface. Some success has been achieved with this method, as indicated in:        3. D. Donskoy et al, “Nonlinear seismo-acoustic landmine detection and discrimination”, J. Acoust. Soc. Amer., 111(6), 2705-2714, 2002.However there appear to be some disadvantages. An LDV is expensive and requires trained personnel to operate it. Scanning is slow with a single LDV. An array of LDVs would be too complex and costly. An LDV has to be carefully aligned with the slope of the ground. Also vibrational coupling between the speaker emitting continuous sound and the LDV can interfere with the LDV signals.Other Acoustic Methods of Detecting Buried Objects        
LDVs are not used in other acoustic methods. Instead microphones detect the echoes of sound pulses from a buried object, as the echoes emerge from the ground. Pulses are used in an attempt to separate incident from reflected sound. An example is                4. L. J. House and D. B. Pape, “Method and Apparatus for Acoustic Energy Identification of Objects buried in Soil”, U.S. Pat. No. 5,357,063, Oct. 18, 1994.Another example is:        5. R. Hickling, “Method and Apparatus for Acoustic Detection of Buried Objects”, U.S. Pat. No. 6,862,252, Mar. 1, 2005.The principal difficulty with these methods is making the pulses short enough to separate incident from reflected sound, within the scale of the structure used in the method.        